1. Field of the Invention
The invention relates to a stator for a rotary electric machine, and more particularly, to a stator for a rotary electric machine, in which ends of phase windings are connected at a neutral point (i.e., one end of one phase winding is connected to one end of another phase winding at a neutral point).
2. Description of Related Art
In a stator of a three-phase rotary electric machine, phase windings are wound by a predetermined winding method. A Δ connection and a Y connection are known methods to connect phase windings together. With a Y connection, ends of phase windings are connected at a neutral point (i.e., one end of one phase winding is connected to one end of another phase winding at a neutral point).
In related art, one end of each phase winding is pulled out, and the pulled out end portions of the phase windings are twisted together, for example, to form a neutral point. For example, Japanese Patent Application Publication No. 2006-121884 (JP 2006-121884 A) describes a structure in which a wound portion formed by a stator coil that is wound through slots in a motor stator is provided, and wires that are supplied with phase currents of the phases protrude out from an upper portion of the wound portion, and neutral lines protrude out from a lower portion of the wound portion. Here, a portion of each of the three neutral lines at the lower portion of the wound portion is stripped of an insulation layer, and these stripped portions are overlapped and welded together.
Also, Japanese Patent Application Publication No. 2004-104996 (JP 2004-104996 A) describes pulling out end portions of three different phase coils as neutral point lead portions, placing the pulled out lead portions along an upper portion of a coil end group, and fixing the lead portions to the coil end group by insulated cloth tubing+adhesive, cloth sheet+adhesive, or partial adhesive or the like.
One method of forming the phase coils is to use a plurality of conductor segments that have been formed in U-shapes or the like beforehand. With this method, the conductor segments are bent into a predetermined shape and end portions of the conductor segments are sequentially connected together by welding. In this case, flat wire having a rectangular cross-section may be used as the conductor segments. In this way, when employing a method of connecting the end portions of a plurality of conductor segments of flat wire together by welding, assembly is easier using a neutral line that is separate from the phase windings, than forming a neutral point by twisting the end portions of the flat wires together. That is, the pulled out end portions of the flat wires are welded and connected together via the neutral line.
For example, Japanese Patent Application Publication No. 2012-29355 (JP 2012-29355 A) describes fixing two neutral lines and three interphase wires that are separate from the phase windings together by adhesive, in a coil end of a stator of a rotary electric machine.
Also, Japanese Patent Application Publication No. 2013-121183 (JP 2013-121183 A) proposes a structure in which a neutral line that is attached by welding to a stator of a rotary electric machine is able to be easily replaced when a temperature detection element provided on the neutral line fails. More specifically, a tip end length of a neutral line weld zone to be welded to a neutral line, of an end portion weld zone of conductor segments that form a stator coil, is made twice as long as a same-phase weld zone.
When a neutral line that is separate from the phase windings is used to connect ends of phase windings at a neutral point (i.e., one end of one phase winding to one end of another phase winding at a neutral point) in a stator for a rotary electric machine, the neutral line is connected and arranged in a state floating between ends that have been pulled out from the phase windings (hereinafter, referred to as “lead ends”). When using a conductor segment, the free end of each phase winding is a free end of the conductor segment, so the neutral line is connected and arranged in a state floating between the two free ends. A resonant frequency of vibration of a portion of the neutral line that is connected in the floating state in this way is a frequency that is lower than a natural frequency of vibration of the stator core and a natural frequency of vibration of an assembly of phase windings wound around the stator core, and tends to resonate with external vibration. For example, in a case such as when the rotary electric machine is mounted in a hybrid vehicle, the neutral line may resonate at the frequency of the engine. If the neutral line resonates, the connecting point between the neutral line and the phase windings may separate, and operation of the rotary electric machine may become unstable.